Wednesday, September 14, 2005

Right Whales, Cyamids and Evolution

The above is a southern right whale. Below is a picture of a cyamid. The genetics of both tell a common story.

Recent genetic studies indicate that there are three species of right whale (North Atlantic, North Pacific and Southern Ocean) which diverged from a single ancestral species some 5-6 million years ago. Scientists studying cyamids (a crustacean parasite on whales)have confirmed the gentic results:

Cyamids were nicknamed "whale lice" by early whalers, who often were infested with real head and body lice. Whale lice are related to crabs and shrimp, and cling to the whales' raised, callus-like patches of skin -- named callosities -- the grooves and pits between callosities, and also to skin in slits that cover mammary glands and genitals. The white whale lice covering the callosities create distinct markings that stand out against the whales' dark skin, making it possible for scientists to distinguish individual whales.


Whale lice infest only whales, just as bird lice infest only birds and human lice infest only people. Recent genetic studies of head and body lice have revealed details of human evolution. Whale lice hang onto the whales throughout their lives, so they share a common ecological and evolutionary history with the whales. Genes from whale lice actually may reveal more about the whales than the whales' own genes do, because the parasites are much more abundant and reproduce more often than whales. As a result, the parasites have much greater genetic diversity and scientists have more mutations to track.

The results mirror the results obtained by studying whale genetics - with one surprising wrinkle:

The same three whale louse species -- Cyamus ovalis, Cyamus gracilis and Cyamus erraticus -- were thought to infest each of the three different species of right whale. But the new study revealed that like the whales, each whale louse species also split into three species, so North Pacific, North Atlantic and southern ocean species of right whales each are infested by three distinct species of cyamid. That tripled -- from three to nine -- the number of cyamid species infesting right whales.


Some 20 million years ago, North and South America were separated by deep seas, but 18 million years ago, undersea volcanism slowly began forming a volcanic island chain. By 3 million years ago, the chain formed solid land, the Isthmus of Panama, linking the two continents. By 5 million or 6 million years ago, the sea between the two continents was so shallow that whales could not swim between the North Pacific and North Atlantic, Rowntree says. Changing circulation patterns established warm currents that discouraged right whales from moving between southern and northern oceans.

"Right whales have such thick blubber they can't cross the equator," Rowntree says. "The waters are too warm. They can't shed heat."

Seger says: "The genetics of whale lice show conclusively that the three species of right whales have been isolated in the North Pacific, North Atlantic and Southern Hemisphere for about 5 million to 6 million years," with a possible range of error from 3.6 million to 9.9 million years.

One other interesting result. Gentic studies of right whales indicated that north Atlantic right whales have lower genetic diversity than southern ocean right whales. The Cyamid study, however, indicates that north Atlantic cyamids have just as much genetic diversity as cyamids on whales in the north Pacific and southern oceans. This indicates that north Atlantic right whale populations were just as large as those in the north Pacific and southern ocean - but suffered recent declines:

North Atlantic right whales have lower genetic diversity than southern ocean right whales. But the new study showed "the genetic diversity of whale lice is virtually as great for the North Atlantic right whale as for the southern right whale, suggesting that historically (but before whaling) the North Atlantic right whale population was comparable in size to that in the Southern Hemisphere," Seger says. "This suggests that the reduced genetic diversity of North Atlantic right whales happened recently, possibly due to whaling, not because the whale population was small even before whaling."

Whale louse populations correlate with population sizes of right whales, so if North Atlantic right whales had small populations before whaling, the diversity of their whale lice would not be as great as those on the southern right whales.

Limited data from North Pacific whale lice suggest right whales also were abundant there before whaling began, in line with early whaling records, Rowntree says.

Small population size can be harmful because it is impossible to avoid inbreeding and an increased risk of genetic disease. The study raises hope for endangered Northern Hemisphere right whales by suggesting that their reduced genetic diversity is a relatively recent phenomenon and perhaps not as severe overall as it appears to be in the particular genes that were studied, Seger says.